Optical techniques for time-steered control of phased array antenna have been under intense study in recent years. These techniques allow for squint-free ultrawideband operation of an antenna array, something not possible to achieve with phase-only steering. A common optical technique for time steering is based on the high-dispersion fiber optic prism (FOP) developed by Frankel et al. herein incorporated by reference. Although successful, this technique suffers from some drawbacks, the most obvious being the use of longs lengths of expensive high dispersion fiber, resulting in significant signal latency and a somewhat large optical control unit.
A nearly latency-free and more compact approach to time-steering can be achieved by replacing the high dispersion fiber with fiber gratings. Several beamforming architectures are in the prior art.
Discrete fiber grating beamformers use an optically tunable delay line formed by uniformly stitching a series of fiber Bragg gratings having discrete but different periods. Each grating is phase-matched to a particular wavelength. An antenna array is then formed by feeding each element with a delay line having a grating spacing proportional to the element position. The drawbacks of this scheme are that it requires many gratings, does not allow continuous beamsteering and it requires accurate, precise spacing of the gratings in order to achieve accurate time delays.
Serially fed discrete fiber grating beamformers use a similar technique to that of discrete fiber grating beamformers, but only use a single discrete grating delay line. The elements of the antenna array are controlled by serially gating the optical signal. This technique still suffers from the same drawbacks as the discrete fiber grating architecture, in addition to severely restricting the types of microwave signals that can be handled.
Chirped fiber grating beamformers are an attractive alternative to overcome the stitching and tuning problems encountered with discrete fiber grating beamformers. When using a chirped fiber grating architecture a continuously tunable delay line can be realized with a single chirped grating because the grating period varies continuously along the grating length. Chirped grating beamformers in which every antenna element is fed by a delay line having a different length and chirp have been proposed, however implementation of this beamformer is difficult because it requires long gratings capable of generating nanosecond-range time delays and the gratings must be proportionally matched in length and chirp.